Self-commissioning NDIR gas sensors

Two detectors of the same kind, each having an identical neutral band-pass filter to the target gas, are installed next to Signal channel and Reference channel detectors as pairs in an AB designed NDIR gas sensor layout, which are called Standard Signal channel detector and Standard Reference channel detector. “Standard” GAMMA is the ratio of Standard signal channel detector output over that of Standard Reference channel detector. “Standard” GAMMA is independent of the measurement Physics of NDIR gas sensors, is dependent only upon the performance characteristics of the sensor component and is also independent of the presence of any amount of target gas in the sample chamber. Consequently, “Standard” GAMMA can be used to proportionally correct and update GAMMA of the sensor as its components age over time thereby rendering such an AB designed NDIR gas sensor self-commissioning or staying accurate over time after initial calibration.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patent application Ser. No. 13/149,738, the disclosure of which is specifically incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is in the field of measuring instruments, and specifically relates to a configuration design and method for an NDIR gas sensor.

BACKGROUND OF THE INVENTION

Output instability or drift over time leading to measurement inaccuracies has long been a major deficiency for gas sensors irrespective of what technology or methodology is used for their conception or realization. Output software correction may alleviate the problem somewhat but it is in many instances inaccurate and not even always applicable. It has long been the objective of many researchers in this field to overcome this problem fundamentally and for good.

Recently the present author in U.S. Pat. No. 8,143,581, the disclosure of which is specifically incorporated by reference herein, advanced the teaching of an Absorption Biased NDIR Gas Sensing Methodology which is capable of eliminating substantially all the NDIR gas sensor output drifts over time without the need for re-calibration. As it turns out, the solution to solving this output drift problems for gas sensors actually lies deeper than the availability of superior NDIR gas sensor types even though they can indeed be designed to be capable of maintaining measurement accuracy over time. The fact of the matter is that people have experienced gas sensor output instability for such a long time in the past that when output stable sensors really come along nobody believes it. Until such time that stable gas sensors become widely available and users begin to consider their performance as trustworthy and truly believable, the real need today must be viewed from a completely different perspective, which is to be able to come up with a fast, inexpensive and simple methodology that can easily check the accuracy of gas sensors and inexpensively re-calibrate them when they are found to be inaccurate.

In U.S. application Ser. No. 13/149,738, filed May 31, 2011, of which this application is a continuation-in-part application, the present author advanced the teaching of a novel Re-calibration Methodology for simply and easily re-calibrating Absorption Biased (AB) designed NDIR gas sensors without the need of standard gases. With the recent advent of the Absorption Biased (AB) gas sensing methodology for realizing NDIR gas sensors whose outputs are significantly drift-free over time and also the advent of a complementing methodology that can check and re-calibrate AB designed NDIR gas sensors simply and easily without the need of standard gases, one would think that the gas sensor industry at large, particularly the HVAC industry, would be relatively satisfied and happily go forward in growing its business. But, unfortunately, this is not the case at all. While the HVAC industry is still trying to deal with their old and on-going problem of sensor inaccuracies over time, already the industry is pushing forward in finding new and better solutions for optimizing energy expenditure and achieving superior comfort level for occupants in buildings. One rather obvious approach widely being investigated and considered everywhere today is the grouping of all sensors in a building together into a computer network. These sensors can actually interact and work with one another in an efficient manner with self-commissioning, self-tuning, self-diagnostic and correction, and even self-configuring features. By so doing the energy requirement for buildings can be reduced to an absolute minimum while the comfort level and safety for occupants in the buildings can also be greatly increased.

No doubt from the standpoint of computer networking hardware and smart software availability today, this approach is clearly workable. However, when all the sensors are to be left alone by themselves to interact with one another over time in buildings, the obvious question to ask is whether these sensors are indeed ready to take on this self-policing task of always staying accurate. In other words, who is there to check whether the outputs of some of these sensors are actually staying accurate over time and if not, what are the consequences for the maintenance status of the buildings and the comfort level and safety of their occupants? Thus, while computer hardware and system networking software may be ready for this futuristic approach to building controls, it is very clear that not all the sensors needed to perform perfectly in this approach are here today to meet the challenge. In particular, gas sensors such as CO₂ and dew point might be relatively accurate over time but for how long before they become inaccurate? But would there be anybody or any mechanism scheduled in the networking controls system to perform the checking or re-calibrating tasks for them? To put it bluntly, until such time that all the required sensors in the networking controls system can be self-commissioning or in other words can render themselves capable of automatically staying accurate all the time, the futuristic building controls approach with the use of computer networking and relevant software to connect all the sensors in the system together working interactively simply will not work.

It is the object of the present invention to advance a configuration design and methodology for AB designed NDIR gas sensors such that they can become self-commissioning or in other words capable of automatically maintaining their measurement accuracy indefinitely over time after initial calibration. This invention is achieved via extending the previously disclosed Absorption Biased methodology of U.S. Pat. No. 8,143,581 and Re-calibration methodology without the need of standard gases (U.S. Ser. No. 13/149,738, Wong) for NDIR gas sensors.

SUMMARY OF THE INVENTION

The present invention is generally directed to a self-calibrating NDIR gas sensor and its use in which an infrared source illuminates a signal channel that is longer than a reference channel while electronics are used to calculate a chosen gas concentration in a sample chamber containing the two channels. The difference in length between the two channels creates an absorption bias between outputs of a signal detector and a reference detector, each of the two detectors having an identical narrow band pass filter with the same Center Wavelength (“CWL”), Full Width Half Maximum (FWHM) and transmittance efficiency at the CWL. A second pair of detectors, called standard detectors, are placed in the two channels, and both of these standard detectors have an identical standard narrow band pass filter with the same Center Wavelength (“CWL”), Full Width Half Maximum (FWHM) and transmittance efficiency at the CWL and the CWL of the standard narrow band pass filter is a neutral wavelength. The electronics of the sensor is calibrated by use of a calibration curve generated by using a normalized ratio of the signal channel output to the reference channel output that starts at unity when there is zero concentration of the chosen gas. The calibration curve is self-calibrated by using a stored standard gamma ratio obtained at a first period of time and a measured standard gamma ratio obtained at a second period of time after the first period of time, the standard gamma ratio being the ratio of a standard signal output from a standard signal detector to a standard reference output from a standard reference detector.

Such an NDIR gas sensor can be made to detect a second gas by including a second signal detector and a second reference detector that function similarly to the signal and reference detector, except that they are designed to detect a different gas. This additional pair of detectors will each have an identical second chosen gas narrow band pass filter with the same Center Wavelength (“CWL”), Full Width Half Maximum (FWHM) and transmittance efficiency at the CWL and will have its own calibration curve generated by using a second chosen gas normalized ratio of the second chosen gas signal output to the second chosen gas reference output that starts at unity when there is zero concentration of the second chosen gas. As was the case with a single gas detection sensor, the second gas calibration curve is self-calibrated by using the stored standard gamma ratio and the measured standard gamma ratio.

The NDIR gas sensor can also be recalibrated by comparing the sample concentration of a gas it is detecting to a second gas measurement of such gas determined by a secondary gas standard and then adjusting the normalized ratio of the signal output to the reference output for the gas based upon a reversed calibration curve algorithm that is a non-linear equation if a difference between the sample concentration of the gas and the second gas measurement exceeds a preselected threshold.

Accordingly, it is a primary object of the present invention to provide an NDIR gas sensor that self-calibrates itself.

This and further objects and advantages of the present invention will be apparent to those skilled in the art in connection with the drawings and the detailed description of the invention set forth below.

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